1. Field of the Invention
This invention generally relates to a cathode ray tube pickup device. More particularly, it relates to a cathode ray tube pickup device best suited for application to facsimile reception printers, computer output printers, etc.
2. Description of the Prior Art
Pickup devices comprising a cathode ray tube using an optical fiber tube provided with an optical fiber plate have heretofore been known in combination with silver salt photography, electrofax or the like. These pickup devices have used photosensitive paper as the copy medium, and this has led to a high running cost and offered problems as to the preservability and weight of the record, the touch of the copy paper, etc.
For this reason, it has been considered to apply optical information from a optical fiber tube to the transfer type electrophotographic system which permits cheaper transfer medium such as ordinary paper or the like to be used as the copy medium.
However, the optical fiber used in the optical fiber plate portion of the optical fiber tube is generally of the construction which comprises, as shown in FIG. 1(a) of the accompanying drawings, a core glass 1 of high refractive index (say, n=1.61), a cladding glass 2 of low refractive index (say, n=1.51) and an outermost light absorbing layer 3, and groups of such optical fiber are arrayed in the manner as shown in FIG. 1(b), with the adjacent optical fibers joined together as by epoxy resin 4. Thus, it is only the core glass portion that actually transmits the optical information from the cathode ray tube to the photosensitive medium, that is, the portions between the core glasses do not transmit the optical information from the cathode ray tube, thus producing dark shadows. As a result, those portions of the optical information applied from the optical fiber tube to the photosensitive medium which correspond to the dark portions of the original information provide dark regions on the photosensitive medium while, in those portions of the optical information which correspond to the light portions of the original information, the core glass portion 1 is light and the portion 2,4 between core glasses is dark, so that the light portions of the original information as imparted to the photosensitive medium are darkened with the light and the dark alternating.
Also, in order to apply the optical information from the optical fiber tube to the photosensitive medium, a uniform contact or a predetermined very slight spacing must be maintained between the optical fiber tube and the photosensitive medium. (Such spacing between the optical fiber tube and the photosensitive medium is imperative because the light emitted from the optical fiber plate has a tendency to become dispersed.) In the above-mentioned silver salt photography or electrofax, the photosensitive medium has sufficient flexibility to readily permit its uniform contact with the optical fiber tube, whereas in the transfer type electrophotographic system, the photosensitive medium is usually in the form of a rigid drum and it is very difficult to maintain a predetermined spacing between such drum and the optical fiber tube, in view of the manufacturing error or the like of the drum. Actually, therefore, it has been considered to improve the optical fiber tube, as shown in FIG. 2, by mounting a correcting optical fiber plate 8 on the surface of the optical fiber plate portion 6 of the optical fiber tube 5 which is opposed to the photosensitive medium 7, thereby setting up an optically uniform condition between the photosensitive medium 7 and the optical fiber tube 5 so as to accomplish a clear exposure. In this case, however, the boundaries between the optical fiber groups do not transmit optical information and thus provide dark shadows, which will appear as noise in the light regions of the applied image. Such adverse effect is relatively small in the case of an optical fiber tube provided with a single optical fiber plate, but in the case of an optical fiber tube additionally provided with a correcting optical fiber plate as described above, the dark regions of the two optical fiber plates irregularly overlap each other to produce a noise image which would make the applied image illegible.
In the image application to the photosensitive medium using the optical fiber tube, as described above in detail, the applied image corresponding to the dark portion of the original information provides a dark region but the applied image corresponding to the light portion of the original information provides a light region mixed with dark noise.
Such image application effected by the use of an optical fiber tube will now be described with respect to the case where it is applied to the typical electrophotographic process disclosed in U.S. Pat. No. 2,221,776 and known as Carson's process.
In FIG. 3, reference numeral 9 designates a photosensitive medium comprising a photoconductive layer 10 and a conductive substrate 11. In FIG. 3(a), the photoconductive layer 10 is uniformly charged with, for example, positive polarity, by a charger 12. Thereafter, as shown in FIG. 3(b), image application is effected by an optical fiber tube 13, whereby the charge remains in the region D corresponding to the dark portion of the original information while, in the region L corresponding to the light portion of the original information, all of the other charge other than that corresponding to the noise is discharged, whereby there is formed an electrostatic latent image. The final copy usually takes the form of a positive image, but in the electrophotographic process now under discussion, a positive image may be formed (1) by effecting a positive image application and developing the electrostatic latent image corresponding to the dark portion of the original information or (2) by effecting a negative image application and developing the electrostatic latent image corresponding to the light portion of the original information. In the case (1) above, however, the electrostatic latent image corresponding to the noise portion is also developed during the development of the electrostatic latent image corresponding to the dark portion of the original information and after all, the final copy obtained will be unclear with the light region being fogged. In the case (2) above, the fog resulting from the noise is eliminated during the development of the electrostatic latent image corresponding to the light portion but such electrostatic latent image would be at a lower or zero potential as compared with the electrostatic latent image corresponding to the dark portion, and such an electrostatic latent image of low potential is undesirable in that, when developed, it could hardly provide a half-tone or would produce an edge effect.